1. Field of the Invention
The present invention relates to an electrophotographic color image forming apparatus, and more particularly to a high voltage switching equipment sequentially supplying a high developing voltage (referred to as a ‘high voltage’ hereinafter) to color developing devices which develop an electrostatic latent image on a photoconductive medium using toner.
2. Description of the Related Art
For example, an electrophotographic color image forming apparatus, such as a color laser printer, a duplicating machine, etc., sequentially develops an electrostatic latent image on a photoconductive medium using color developing devices, i.e., a yellow toner developing device, a magenta toner developing device, a cyan toner developing device and a black toner developing device, and then transcribes and prints a visible image on a paper.
In a printing process mentioned above, a developing process adopts a method where a toner of a developing device is transferred to a photoconductive medium by a voltage difference between the photoconductive medium and the developing device and in order to perform such method, a high voltage should be supplied to one selected developing device or the above four color developing devices in sequence.
A typical example for a high voltage switching equipment of a developing device, adopted in a general electrophotographic color image forming apparatus, is illustrated in FIGS. 1 and 2, which will be briefly described in the following.
In FIGS. 1 and 2, the high voltage switching equipment includes a photoconductive drum 80, a developing device 82, a driving unit 83. Before description, it should be noted that a general color image forming apparatus has four color developing devices as shown in FIG. 1. Since the four developing devices have the same structure and operate in the same manner, description for only one developing device will be made for convenience.
As shown in FIG. 1, the four developing devices 82 are installed to contact the photoconductive drum 80 selectively or sequentially. Such respective developing device 82 has a developing roller 90 and a toner supplying roller 92 as shown in FIG. 2.
The driving unit 83 has a motor 86 and a cam 84 and is disposed in a rear end of each developing device 82. The driving unit 83 moves a relevant developing device 82 back and forth with respect to a side of the photoconductive drum 80, thereby the developing roller 90 become close on the photoconductive drum 80 so that toner on the developing roller 90 is transferred to the photoconductive drum 80. A curved contact 94 is disposed at the rear end of each developing device 82 to contact the cam 84 of the driving unit 83.
In the meantime, a high voltage terminal 88 generating a voltage difference necessary for development at the photoconductive drum 80 and the developing roller 90 is fixedly installed in a frame not shown. Therefore, when the developing device 82 is moved forth toward the side of the photoconductive drum 80 by the driving unit 83, a shaft of the developing roller 90 and the high voltage terminal 88 come in contact with each other, whereby the high voltage is supplied and the toner on a relevant developing device 82 is transferred to the photoconductive drum 80. After that, the developing device 82 is moved back by a restoring spring installed in its rear end, and another developing device 82 is moved forth by the driving unit 83, whereby development by toner of another color is performed. By such series of successive operations, the color developing devices are moved back and forth in sequence and the electrostatic latent image on the photoconductive drum 80 is developed using the color toners.
But the above-mentioned general high voltage switching equipment for the developing device adopted in the color image forming apparatus, operates in a manner such that the developing device 82 is moved back and forth with respect to the side of the photoconductive drum 80 using the driving unit 83 having the motor 86 and the cam 84. Accordingly, vibration due to a movement of the developing device 82 and a load variation upon a rotation of the photoconductive drum 80 are generated since the developing device 82 contacts the photoconductive drum 80 periodically. Such factors cause a change in a velocity of the photoconductive drum 80 in the long run, thereby generating a jitter that is fatal to an image quality of a color image.
Further, the above mentioned high voltage switching equipment of a conventional movement type developing device maintains a proper developing nip to obtain a stable image quality only if a movement distance of the developing device is always constant. But when the movement distance of the driving unit 83 is changed due to deterioration and abrasion of the driving unit 83 upon use for a long period of time and the movement distance of the developing device 82 is changed, a constant developing leaf can not be maintained accordingly, whereby a problem of deterioration in the image quality occurs.
Still further, the high voltage switching equipment of the conventional developing device requires the motor 86 or the cam 84 necessary for movement of the developing device 82, and a sensor for sequentially driving color developing devices. Therefore, a problem that increases a manufacturing cost inevitably occurs.